The present invention relates to a phase synchronizing circuit to efficiently reproduce a reference carrier wave from multi-level, multi-phase modulated carrier waves.
The transmission of digital-signal-modulated carrier waves by the use of multi-phase phase shift keying (PSK) modulation is already applied for practical application. At the same time, with an eye to more efficient utilization of the frequency band, the feasibility of so-called multi-level, multi-phase, superposition-modulated signal transmission systems, by which both the phase and the amplitude of the carrier are concurrently modulated by digital signals, is now under study. One such multi-level, multi-phase, superposition-modulated signal transmission systems is known the 16 quadrature amplitude modulation (QAM) system.
This 16 QAM system, though greater in information transmitting capacity, involves difficulties in circuit structuring. Its application to a carrier wave reproducing circuit, in particular, gives rise to new difficulties, other than those encountered with multi-phase phase-shift-keyed (PSK) carrier waves. While the output signal vectors of a multi-phase PSK carrier wave have equal amplitudes and equal phase differences, those of a 16 QAM wave have mutually different amplitudes and different phase differences. Therefore, unlike the case of multi-phase PSK carrier wave reproducing circuits, means to simply frequency-multiply the input signal would have no place in 16 QAM carrier wave reproducing circuits. One example of a 16 QAM carrier wave reproducing circuit example of which is the phase synchronizing circuit described in my U.S. Pat. No. 4,099,130. The phase synchronizing circuit consists of means for detecting the phase position of each modulated signal by the use of demodulated signals; means for driving a phase modulator, an amplitude modulator or an analog switch with the output signal of said detecting means as control signal and thereby generating four modulated signal vectors, one of which exists in each of the four quadrants of the vector diagram representing the phases and amplitudes of the modulated carrier wave, into one signal vector to equivalently convert a 16 QAM modulated carrier wave into a 4 PSK modulated carrier wave; and 4 PSK phase synchronizing means.
Such a phase synchronizing circuit is useful for constructing a carrier wave reproducing circuit for 16 QAM, and especially so as a base band processing phase synchronizing circuit, which is easy to handle. They, however, have the following disadvantages. First, an analog switch is an indispensable circuit therein, but it is difficult to realize an analog switch which is free from DC drift, satisfactory in input/output characteristics and moreover fast acting. Secondly, although the control signal for the analog switch should coincide with the input signal in bit timing, the control signal, because it is made from a data signal which has once gone through discrimination, lags behind the input signal by half a bit repetition period. Accordingly, an analog delay line is required to delay the input signal by half a bit repetition period, but no well performing analog delay line is currently available, and the inadequacy of this line constitutes a factor to invite performance deterioration.